A substantial portion of a seated person's weight is born upon the seat bones, (i.e., ischial tuberosities). For people who are restricted to wheelchairs, or otherwise to sitting positions, this area is particularly prone to the formation of pressure sores or decubitis ulcers. Patients who are victims of stroke, hip fracture, spinal cord injury and brain injury, as well as paraplegics and geriatrics are particularly susceptible to such sores. These sores are difficult to heal and require expensive prolonged recovery periods.
Attempts have been made within the prior art to provide for seating systems which cushion these areas and lessen capillary blood pressure between the seating surface and the seat bones, where pressure is otherwise likely to build up.
One such example of a prior art system is presented in U.S. Pat. No. 4,726,624 and its reissue patent. In this instant, the prior art utilizes a rigid base having a fluid filled envelope which extends over the entire base and in particular, up over side rims of a seating depression so as to cause a contact between the rigid base and the soft tissue of the user to encourage the flow of viscous fluid material underneath the seat bones of the user. Notable disadvantages of this particular system are the weight of the cushion, and the expense required by the large fluid envelope, as well as the sometimes troublesome reliance on the soft tissue used as a positioning means and means to interact indirectly with the rigid side rim of the base cushion so as to cause fluid flow down into the base of the cushion. In cases of people confined to wheelchairs, the soft tissue is not always of a shape or tone to enable reliance on this particular interface.
It is, thus, an object of the present invention to provide a seat assembly having a foam cushion which provides for primary positioning and support of the user. A further object is to provide a more circumscribed area of pressure compensation directly underneath the seat bones of the chair occupant and not extending over the entire base (i.e., less than half, preferably less than one fourth, preferably about 1/7 to 1/8 of the surface area of the base). A further object of the invention is to provide a pressure compensation envelope which is seamed so as to inhibit undesirable fluid migration, and yet to enable a limited amount of fluid transition so as to minimize the possibility of rupture at seams.
Finally, an object of the invention is to provide a simple assembly having only a few parts which can be used optionally with a rigid or sling type wheelchair seat.